N. J. Strausfeld – författare

The present volume covers the physiology of the visual system beyond the optic nerve. It is a continuation of the two preceding parts on the photochemistry and the physiology of the eye, and forms a bridge from them to the fourth part on visual psychophysics. These fields have all developed as independent speciali­ ties and need integrating with each other. The processing of visual information in the brain cannot be understood without some knowledge of the preceding mechanisms in the photoreceptor organs. There are two fundamental reasons, ontogenetic and functional, why this is so: 1) the retina of the vertebrate eye has developed from a specialized part of the brain; 2) in processing their data the eyes follow physiological principles similar to the visual brain centres. Peripheral and central functions should also be discussed in context with their final synthesis in subjective experience, i. e. visual perception. Microphysiology and ultramicroscopy have brought new insights into the neuronal basis of vision. These investigations began in the periphery: HARTLINE''S pioneering experiments on single visual elements of Limulus in 1932 started a successful period of neuronal recordings which ascended from the retina to the highest centres in the visual brain. In the last two decades modern electron­ microscopic techniques and photochemical investigations of single photoreceptors further contributed to vision research.

The present volume covers the physiology of the visual system beyond the optic nerve. It is a continuation of the two preceding parts on the photochemistry and the physiology of the eye, and forms a bridge from them to the fourth part on visual psychophysics. These fields have all developed as independent speciali­ ties and need integrating with each other. The processing of visual information in the brain cannot be understood without some knowledge of the preceding mechanisms in the photoreceptor organs. There are two fundamental reasons, ontogenetic and functional, why this is so: 1) the retina of the vertebrate eye has developed from a specialized part of the brain; 2) in processing their data the eyes follow physiological principles similar to the visual brain centres. Peripheral and central functions should also be discussed in context with their final synthesis in subjective experience, i. e. visual perception. Microphysiology and ultramicroscopy have brought new insights into the neuronal basis of vision. These investigations began in the periphery: HARTLINE'S pioneering experiments on single visual elements of Limulus in 1932 started a successful period of neuronal recordings which ascended from the retina to the highest centres in the visual brain. In the last two decades modern electron­ microscopic techniques and photochemical investigations of single photoreceptors further contributed to vision research.

The "functional" in the title of this book not only reflects my personal bias about neuroanatomy in brain research, it is also the gist of many chapters which describe sophisticated ways to resolve structures and interpret them as dynamic entities. Examples are: the visualization of functionally identified brain areas or neurons by activity staining or intracellular dye-iontophoresis; the resolution of synaptic connections between physiologically identified nerve cells; and the biochemical identification of specific neurons (their peptides and transmitters) by histo- and immunocytochemistry. I personally view the nervous system as an organ whose parts, continuously exchanging messages, arrive at their decisions by the cooperative phenome­ non of consensus and debate. This view is, admittedly, based on my own ex­ perience of looking at myriads of nerve cells and their connections rather than studying animal behaviour or theorizing. Numerous structural studies have demonstrated that interneurons in the brain must receive hundreds of thousands of synapses. Many neurons receive inputs from several different sensory areas: each input conveys a message about the external world and possibly also about past events which are stored within the central nervous system. Whether an interneuron responds to a certain combination of inputs may be, literally, a matter of debate whose outcome is decided at the post­ synaptic membrane. A nerve cell responding to an overriding command is possibly a rare event.